During a dock loading operation, cargo can be moved between a loading dock and a truck parked in front of the dock either manually or through use of material handling equipment, such as a fork lift truck. During the loading operation, the rear end of the truck, which is located against the dock, completely encloses the doorway in the dock so that the loading operation is not visible to the truck driver. Consequently, there have been instances when a truck driver, thinking that the loading operation has been completed, has pulled away from dock with the result that personnel or material handling equipment could be injured or damaged.
Various types of devices have been used in the past to prevent a truck from pulling away from a loading dock during a loading operation. The most common type of restraining device is a wheel chock in which chocks are manually placed behind the wheels on the truck. However, wheel chocks are frequently lost or misplaced, and as a result, they are not available for use at the loading dock. In other situations, the dock attendant or truck driver may be negligent in manually positioning the wheel chocks.
As a result of the problems associated with manual wheel chocks, automatic truck restraining devices have been proposed. One common form of truck restraint is one that is either mounted on the front face of the loading dock or on the driveway in front of the dock face and is employed to engage the ICC bar at the rear end of the truck. An ICC bar is a structural bar or beam that is located at the rear end of the truck, below the truck bed, to prevent automobiles from under-riding the truck in the event of a rear end collision. Truck restraints of this type generally include either a hook or a blocking member which is automatically actuated through controls on the loading dock to engage the ICC bar, or to move to a blocking position outwardly of the ICC bar, to prevent accidental movement of the truck away from the loading dock.
The conventional truck restraint is a complicated mechanism due to the fact that the truck restraint must be able to accommodate various types of trucks and various configurations of ICC bars. More particularly, ICC bars can vary from about 12 inches to 30 inches above the ground and the truck restraint must be able to engage the ICC bar at all of the working levels. Further, during a loading or unloading operation, the truck bed will tend to float. For example, as cargo is removed from the truck bed, the truck springs will relax with the result that the truck bed will float upwardly. Conversely, as cargo is applied to the truck bed, the truck bed will float downwardly. Consequently, the truck restraint must be able to accommodate float of several inches and still maintain engagement with the ICC bar.
U.S. Pat. No. 4,969,792 describes a further type of truck restraining device which takes the form of an automatic wheel chocking mechanism. In this patent, a pair of parallel troughs are formed in a support platform which is mounted for pivoting movement within a pit formed in the driveway in front of the loading dock. A wheel chock is mounted for movement within each trough between a recessed position, where the chock is located beneath the upper surface of the platform, to an operative or chocking position, where the chock will engage the truck wheel to prevent the truck from moving away from the loading dock.
As described in the aforementioned patent, the wheel chocks are moved between the recessed and operative position by a cylinder-piston arrangement in which the piston is fixed to the platform and the cylinder is movable relative to the piston and carries a chain which is interconnected with the wheel chock. With this construction, extension of the cylinder-piston unit will move the wheel chock from its recessed or storage position to the operative or chocking position.
The use of a cylinder-piston unit has certain disadvantages in that the reciprocating path of movement of the wheel chock is limited to the stroke of the cylinder. Therefore, the construction in the aforementioned patent incorporates a chain drive mechanism with the cylinder in order to increase the path of travel of the wheel chock over the stroke of the cylinder-piston.
The use of a cylinder-piston unit also requires that hydraulic pressure be continuously maintained in the cylinder when the wheel chock is in the operative or chocking position. This continuous application of hydraulic pressure over extended periods can result in wear on the hydraulic system and possible leakage of hydraulic fluid.